In a typical cellular radio system, user equipment unit nodes (UEs) (also known as wireless terminals or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a RAN node, e.g., a radio base station (BS), which in some networks is also called a “NodeB” or enhanced NodeB “eNodeB.” A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Wireless communication systems are often characterized by time-varying interference environment. Cognitive radio as a new paradigm uses knowledge of interference conditions to improve aspects of radio performance such as throughput or coverage.
In parallel with development of cognitive radio, there has been a growing demand to receive multiple carriers simultaneously. Carrier aggregation (CA) is an example of multiple carrier reception where multiple carriers are transmitted and received from a single UE. Carrier aggregation is being considered for use in the 3GPP. In particular, the LTE-Advanced standard considers intra-band non-contiguous carrier aggregation where multiple carriers are separately placed within a band. Software defined radio is another example where multiple wireless systems are supported by a single UE simultaneously. A challenge of multiple carrier reception is to share as much radio hardware as possible between reception/transmission chains for the different carriers, for example, to reduce power consumption and/or hardware cost.
As a radio architecture of a UE supporting multiple carrier reception, a direct conversion receiver [DCR] or a double conversion receiver may be used. These two radio architectures may have different advantages and disadvantages. A double conversion receiver may enable cost-efficient and hardware-efficient implementation, because multiple carriers may share the RF front-end (e.g., low-noise amplifier and mixers) and (portions of) IF mixing stages. A double conversion receiver, however, may be more susceptible to IQ imbalance and/or harmonic mixing. A double conversion receiver may thus require more careful radio design.
Both IQ imbalance and harmonic mixing may cause an undesired signal(s) to interfere with a desired signal(s). Interference due to IQ imbalance and/or harmonic mixing, however, may involve different signals, and if they involve the same signal, the resulting interference level may be different. IQ imbalance may cause the image signal (i.e., a signal separated from the RF mixer frequency by the IF mixer frequency on the opposite side of the desired signal) to interfere with the desired signal. The interference level may be determined by the image rejection ratio, which is given as a function of gain and phase imbalance of the RF mixing stage. On the other hand, harmonic mixing may involve primarily the odd harmonics (i.e., the signals distant from the RF mixer frequency by odd multiples of the IF mixer frequency), and even harmonics to a much lesser extent. The interference level may be determined by a harmonic rejection ratio, which is determined by the architecture of harmonic rejection mixer (and also the relevant gain and phase unbalance). Accordingly, the SINR of the desired signal may depend on radio parameters (e.g., image rejection ratio, harmonic rejection ratio etc.) as well as interference conditions.
One approach to mitigating effects of interference is to select radio frequency carriers with higher SINR (Signal-to-Interference plus Noise-Ratio) to improve reception. The receiver measures SINRs of ail available channels and feeds the information back to the transmitter. The transmitter uses the RF carriers with higher SINR, based on the information from the receiver. This is often referred to as cognitive radio in the literature, and it can be seen as an adaptation based on interference condition, implementation, however, requires additional signaling overhead (to communicate SINR information and channel selection), and carriers with desirable SINR may not always be available.